A document comprising a sheet, such as film or paper bearing alphanumeric or graphical information, prepared by handwriting, typewriter, photocopier, printer, telex, and fax, can be reproduced by a great number of photocopiers without the consent of the owner. The illegal copier of documents is more and more frequent in the fields of economy, science, politics or the military and can cause immense financial or moral damages.
The effort to protect official documents, such as banknotes, checks, bonds, identity cards and passports against falsification has led to technical solutions based on the utilization of counterfeit deterrents, such as filigranes, threads, fibers, colored or metallized or magnetized elements. The state of the art is described in British patent 1 127 043. Unfortunately the security papers prepared according to these methods do not protect the documents, or protect them only insufficiently against photocopying.
Several methods of preventing copyability of documents have been patented and, in a few cases, commercialized. A paper coated with a cacao-colored layer has been produced by Calspan and Ludlow Paper Co., Needham Heights, Mass., USA. U.S. Pat. No. 3,597,082 mentions photochromic pigments to avoid photocopying. The French patent 2 177 292, assigned to IBM, uses special inks printed on a masking background. U.S. Pat. No. 4,578,298 describes a selfadhesive film protecting documents against photocopying. This product comprises a colored film coated on a semi-transparent, vacuum-metallized film. Nocopy International Inc. has commercialized a security paper based on dark pigments covering the sensitivity spectrum of modern photocopiers.
All these products are expensive to manufacture or limited in their application, or else have poor contrast.